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IBM enters the field in Pentium battle

By Charles Arthur

COMPUTER giant IBM stepped up pressure on the chipmaker Intel last week. It claimed that the flaw in Intel’s Pentium microprocessor could produce an error every few weeks on average, instead of once every 27 000 years of use as Intel insists. IBM said people using spreadsheets for financial calculations were particularly at risk.

But Intel was quick to rebut the claims. “You can always contrive situations that force this error,” said Andrew Grove, Intel’s chief executive. “Just as, if you know where a meteor is going to land, you can go there and get hit.” The flaw in its chip leads to errors beyond the fourth significant digit when dividing particular pairs of numbers.

IBM’s analysis was released onto the Internet on its World-Wide Web page (at http://www.ibm.com) late last week. It followed hard on the heels of the company’s announcement that it would no longer ship computers containing the Pentium chip until it was supplied with bug-free versions. IBM is one of Intel’s biggest customers.

But Intel said it would stick to its policy of only replacing flawed versions of the chip for scientists and engineers who can show that their work might be affected by the fault (Technology, 10 December). Other PC-clone makers such as Dell, Compaq, Gateway 2000, AST and Digital, said they would continue to sell Pentium-based machines. Microsoft announced that it had written a software “fix” for its Excel spreadsheet that should bypass the flaw.

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IBM’s move to stop Pentium shipments might eventually help the PowerPC chip, a competitor to Pentium developed jointly by IBM, Motorola and Apple. But IBM cannot benefit from Pentium users switching to PowerPC machines because as yet it has no desktop versions for sale.

Intel’s own analysis suggested that the average Pentium user would encounter the flaw once in every 9 billion division operations. Assuming a computer made 1000 such divisions per day, it would only make a mistake as a result of the pentium flaw once in 27 000 years. But scientists at IBM’s Thomas J Watson Research Center in New York estimated that the chance of a mistake is closer to one in every 100 million divisions. They also said that the average spreadsheet does about 5000 divisions each second during calculations. At that rate, the computer could have performed well over 4 million divisions after only 15 minutes. At 15 minutes of use per day, that would mean a miscalculation every 24 days.

“Hypothetically, if 100 000 Pentium customers were doing 15 minutes of calculations every day, we could expect 4000 mistakes to occur each day,” said IBM. Financial users are particularly at risk, it adds, because calculations involving four-figure numbers of the form £xx.xx can easily fall into a pattern in which the flaw shows up.

Intel responded by saying that analysis of a variety of spreadsheet users at its own offices backed up its estimate of 1000 divisions per day, and that four-figure numbers are not statistically any more likely than others to lead to errors.

The bug is in the part of the chip dealing with floating-point arithmetic. According to Christopher Duhon of John Hopkins University Applied Physics Laboratory, the Pentium relies on a “lookup table” as a short cut for certain calculations. During the design of the chip, five entries were accidentally omitted from the table, leading to errors whenever a calculation refers to any of those numbers.